1. Field of the Invention
This invention relates to an immersion nozzle for continuously casting molten metal, particularly clean molten steel having less non-metallic oxide inclusions, bubbles and powdery inclusions and a method of continuously casting molten metal by using this immersion nozzle.
2. Related Art Statement
In the continuous casting of molten steel, the immersion nozzle has hitherto been used when molten steel is poured from a tundish into a mold. A typical example of this immersion nozzle is shown in FIG. 1, wherein the sectional area of the passage for passing molten steel through the immersion nozzle 1 is designed to become smaller than the total area of discharge ports formed in the opposite sides of the immersion nozzle 1 from a viewpoint of the restriction on the size of the mold for continuously casting into a slab (including bloom, beam blank, billet and the like). Therefore, when molten steel flowing down through the passage of the immersion nozzle at a high speed is discharged from the wide discharge port into the mold, the down component of the molten steel stream remains in the mold, non-metallic inclusions such as alumina and the like and bubbles entered with the down-flow molten steel deeply penetrate into molten steel and are trapped by the resulting solidification shell to degrade the quality of the continuously cast slab. In the curved-type continuously casting machine, there is particularly caused a problem that the non metallic inclusions and bubbles once deeply caught in molten steel are trapped below the lower surface of the solidification shell without floating up to meniscus portion and generate drawbacks such as slivers, blisters and the like on the surface of the steel product such as sheet and pipe after rolling.
As a countermeasure for preventing the occurrence of the down-flow component of molten steel stream, there are mentioned the following.
It is considered to make the area of the discharge port small in the immersion nozzle. In this case, however, the discharge speed of molten steel becomes large. As a result, molten steel discharged from the immersion nozzle collides to the narrow side of the mold to be changed into a down flow thereof and consequently there is a possibility that the non-metallic inclusions such as alumina and the like and bubbles are trapped by the solidification shell, resulting in the degradation of the quality of steel product.
Further, it is considered to arrange a regulating vane for stopping the down-flow component of molten steel stream. However, there is a problem that the regulating vane is not durable to the flowing of high-temperature molten steel at high speed.
Moreover, it is considered to make large the sectional area of the passage for molten steel in the immersion nozzle. In this case, however, the thickness of the mold is restricted, so that it is difficult to charge molten steel into a portion between the mold and the outer surface of the immersion nozzle.
In order to solve the above problems, Japanese Patent laid open No. 61-23558 and Japanese Utility Model laid open No. 55-88347 disclose a technique for preventing the penetration of molten steel stream into un solidified region by improving the immersion nozzle.
FIG. 2 shows an immersion nozzle 2 described in Japanese Patent laid open No. 61-23558 wherein the bottom of the nozzle is curved in semi-spherical form and three or more discharge ports 3 per one side of the nozzle are formed therein for discharging molten steel. FIG. 3 shows an immersion nozzle 4 described in Japanese Utility Model laid open No. 55-88347, wherein two discharge ports 5 opposed to each other and opening in a horizontal or obliquely upward direction are arranged in the lower end portion of the nozzle and two discharge ports 6 opening in an obliquely downward direction are arranged just above the ports 5, whereby streams of molten steel discharged from these ports are collided with each other.
In these immersion nozzles, however, as the flowing speed of molten steel through the inside of the nozzle becomes larger, molten steel is discharged from only the ports at the lower end portion of the nozzle, so that there is a problem that the down flowing of molten steel stream is accelerated to make large the penetration depth of molten steel. On the other hand, there is a fear that negative pressure is generated at the upper discharge ports and mold powder is absorbed in molten steel to undesirably increase the amount of powdery inclusion.